Process for the modification of elastomers with surface interpenetrating polymer networks and elastomers formed therefrom

ABSTRACT

A process for forming a surface modification on a polymer substrate and polymer substrates having such surface modifications. The process comprises the steps of absorbing a swelling monomer into the polymer substrate for a period of time in order to swell the polymer substrate; removing the swollen polymer from the swelling monomer; transferring the swollen polymer to a reaction mixture containing at least one functional monomer; polymerizing the functional monomer in the reaction mixture containing the swollen polymer substrate for a period of time; and removing the polymer from the reaction mixture. Because the surface modification produced by the process is a surface interpenetrating polymer network, the process is not sensitive to the reactive groups located on the surface of the polymer substrate. Further, the surface interpenetrating network bonds to the polymer substrate through caternary connections or other forms of chain entanglement and this is quite stable. Polymer substrates having the surface modification of the present invention are capable of having a surface modification agent, such as heparin, adhere to the surface of the polymer substrate.

The present application is a continuation of U.S. patent applicationSer. No. 09/876,572, filed on Jun. 7, 2001, now abandoned, which is adivisional of U.S. patent application Ser. No. 08/867,269 filed on Jun.2, 1997 now U.S. Pat. No. 6,011,082.

FIELD OF THE INVENTION

This invention relates in general to a process for forming a surfacemodification on a polymer and relates in particular to a process forforming a surface interpenetrating polymer network on polymers and tothe polymers having such surface interpenetrating polymer networks.

BACKGROUND OF THE INVENTION

Polymers are often used in medical device application due to the factthat they are easy to process, they have good mechanical properties, andthey have an acceptable level of biocompatability. In certain medicaldevice applications, the surface of the polymer will be in contact withthe cells and fluids of the body. In these applications, it is necessarythat the surface of the polymer have certain beneficial properties inorder to be in contact with the cells and fluids of the body. In certainapplications and with certain polymers, the surface of the polymer willnot embody these certain beneficial characteristics. In such cases, itmay be necessary to modify a thin surface layer of the polymer so thatit may embody these beneficial characteristics, while at the same timemaintaining the beneficial characteristics of the bulk properties of thepolymer.

Numerous types of polymer surface modifications and methods forpreparing such modifications are known. Such existing surfacemodification methods on polymers include gas plasma, radiation grafting,photoinduced grafting, sol-gel process, surface etching, and polyamineadsorption. Although these existing surface modification techniques areadequate for their purposes, they each have their drawbacks. Forinstance, the gas plasma technique tends to yield non-uniform surfaceson polymers such as silicone, which would increase, rather than reduce,cell adhesion. Ionizing radiation may weaken and discolor the polymermaterial, which is a significant drawback with silicon intraocularlenses. In photoinduced grafting, a coupling agent reacts directly withthe polymer substrate surface, and thus the process is sensitive to theparticular reactive groups located on the surface of the polymersubstrate. The sol-gel process creates a modified surface that lackslong-term stability. Surface etching cleaves the polymer backbone, whichmay weaken the surface structure of the polymer.

Another method of making a surface modification to a polymer is tochemically bond a coupling agent directly to the surface of the polymer.This method requires that the coupling agent bond directly to a reactivegroup on the surface of the polymer substrate. Thus, because thecoupling reaction is dependent on the reactive groups located on thesurface of the polymer substrate, the particular reaction conditions andcoupling agents will be dependent on the particular reactive group.

Another known surface modification technique is the use of bulkinterpenetrating polymer networks. An interpenetrating polymer networkis a combination of two polymers in the network form, at least one ofwhich is polymerized in the presence of the other. Bulk interpenetratingpolymer networks are synthesized by polymerization through the bulk ofthe polymer. For instance, European Patent No. 643,083 discloses a bulkinterpenetrating polymer network form polydimethylsiloxane andpolyacrylic acid for fabricating soft contact lenses. Further, U.S. Pat.No. 5,426,158 discloses contact lenses made from a bulk interpenetratingpolymer network.

The bulk interpenetrating polymer networks, although adequate for theirintended use, have drawbacks of their own, For instance, in bulkinterpenetrating polymer networks, the polymerization initiator isuniformly distributed throughout the bulk of the polymer to be modified.Polymers that have bulk interpenetrating polymer networks formed thereinhave different physical properties than the same polymer without thebulk interpenetrating polymer networks, because networks are createdthrough the entirety of the polymer. Thus, for example, an untreatedclear polymer that has a bulk interpenetrating polymer network may becloudy throughout, and thus not suitable for an optical application.Further, in polymers having bulk interpenetrating polymer networks, thefunctional monomers used in the polymerization process are mixed intothe entire bulk of the polymer, and therefore only part of thesemolecules are available for functioning on the surface of the polymer.

Accordingly, it will be appreciated from the foregoing that there is adefinite need for a process whereby the surface of a polymer can bemodified to have certain desired properties, while at the same timemaintaining the desirable physical properties of the polymer. One of thedesired properties resulting from the surface modification should be theability of the surface of the polymer to couple with a surfacemodification agent, such as heparin. The process should provide for aninterpenetrating polymer network which occurs only on the surface of thepolymer, rather than in the bulk of the polymer. The coupling agent ofthe interpenetrating polymer network should not be chemically bound toany reactive groups on the polymer substrate so that the process offorming the surface modification is not sensitive to the particularreactive groups located on the polymer substrate. Further, the processshould yield uniform surfaces on the polymer, should not weaken ordiscolor the polymer material, should be relatively simple andinexpensive, and should provide a surface modification that haslong-term stability. The present invention meets these needs.

SUMMARY OF THE INVENTION

In accordance with the present invention, a process for forming asurface modification on a polymer is provided. The process modifies theproperties of the polymer at its surface. The process can be used toincrease the ability of a surface modification agent, such as heparin,to adhere to the surface of the polymer. The surface of the polymer ismodified by a surface interpenetrating polymer network, which providesfor the indirect bonding of the polymer network with the surface of thepolymer substrate in a caternary connection or other form of chainentanglement. There are no direct bonds to any reactive groups locatedon the surface of the polymer substrate, so the process is not sensitiveto the particular reactive groups located on the surface of the polymersubstrate. Further, the surface modification of the present inventionaffects only the surface of the polymer, rather than the bulk of thepolymer. Thus, the desirable physical properties of the bulk polymer aremaintained. The process also yields uniform surfaces on the polymer,does not weaken or discolor the polymer material, is relatively simpleand inexpensive, and provides a surface modification that has long-termstability.

The present invention provides a process for the surface modification ofa polymer involving the use of surface interpenetrating polymernetworks. An interpenetrating polymer network is a combination of twopolymers in the network form, at least one of which is polymerized inthe immediate presence of the other. An interpenetrating polymer networkis distinguished from simple polymer blends, blocks, or grafts in twoways: (1) an interpenetrating polymer network swells, but does notdissolve in solvents, and (2) creep and flow are suppressed.

In general, the interpenetrating polymer networks of the presentinvention are prepared by the introduction of a swelling monomer intothe surface of a polymer substrate in order to swell the polymersubstrate at its surface. The swelling occurs in a solvent, although thesolvent may not be necessary, because the monomer should be able todiffuse into the substrate surface with or without the aid of a solvent.The swelling process occurs at a particular temperature and for aparticular time period. The swelling monomer is then catalyzed in thepresence of an initiator that has been introduced into the reactionmixture. The swelling monomer may be a crosslinking monomer, afunctional monomer, or a combination of both.

In another embodiment of the present invention, the polymer substratemay be removed from the surface modification process of the presentinvention following swelling and placed in a reaction medium containinga functional monomer, and, in some cases, a solvent. The functionalmonomer is then polymerized in a polymerization reaction which may beinitiated by a catalyst, or by UV radiation, heat, or ionizationradiation. The polymerization of the swelling monomer and the functionalmonomer occurs at a particular temperature for a particular period oftime. Upon initiation, polymerization proceeds in the solution resultingin a soluble polymer. On the surface of the polymer, however, thepolymerization results in an interpenetrating polymer network due to thepresence of the swelling monomer, which polymerizes together with thefunctional monomer at the surface interface of the polymer substrate.

The surface interpenetrating network formed on the polymer substrate isquite stable. The bonding between the functional monomer and the polymeris indirect, in that caternary connection and other forms of chainentanglement are responsible for the bonding of the coupling agent andthe polymer. Because the functional monomer does not chemically reactwith the substrate, this interpenetrating polymer network process israther insensitive to the substrate surface, as long as the surfaceswells to a certain extent. Thus, in order to break the surfaceinterpenetrating polymer network, a covalent bond on theinterpenetrating polymer must be broken. Even if such a covalent bond isbroken, the interpenetrating polymer will still be entrapped within thesurface of the polymer substrate and thus the surface modification tothe polymer substrate will remain virtually intact.

The surface modification process of the present invention is useful formodifying the surface of a silicon polymer. Thus, silicone intraocularlenses, silicone contact lenses, silicone particles for a chromatographycolumn, and other medical devices may be formed. Further, the surfacemodification process is useful for permitting a surface modificationagent, such as heparin, to adhere to the surface of a silicon lenshaving the surface interpenetrating polymer network of the presentinvention, for instance.

Other objects, features, and advantages of the present invention willbecome apparent from a consideration of the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the traditional surface modification methodof bonding one end of a coupling agent to a reactive group on a siliconelens surface;

FIG. 2 is a diagram showing one of the surface interpenetrating polymernetwork molecules of the present invention;

FIG. 3 is a graph showing the attenuated total reflectance infraredspectrum (ATRIR) for an unmodified silicone substrate;

FIG. 4 is a graph showing the attenuated total reflectance infraredspectrum (ATRIR) for a silicone substrate modified by the process of thepresent invention; and

FIG. 5 is a graph showing the attenuated total reflectance infraredspectrum (ATRIR) for a silicone substrate modified by the process of thepresent invention that has been extracted with ethanol for three days.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is embodied in a process for providing a surfacemodification to a polymer in the form of an interpenetrating polymernetwork on a polymer substrate. The process is useful in that it can beused to increase the ability of a surface modification agent, such asheparin, to adhere to the surface of the polymer. The surfaceinterpenetrating polymer network provides for the indirect bonding ofthe polymer network with the surface of the polymer substrate in acaternary connection or other form of chain entanglement. Because thereis no direct reaction with the reactive groups of the polymer substrate,the process of the present process is not sensitive to the particularreactive groups located on the surface of the polymer substrate.Further, the surface modification of the present invention affects onlythe surface of the polymer, rather than the bulk of the polymer. Thus,the desirable physical properties of the bulk polymer are maintained.The process also yields uniform surfaces on the polymer, does not weakenor discolor the polymer material, is relatively simple and inexpensive,and provides a surface modification that has long-term stability.

In accordance with the present invention, a polymer substrate isprovided on which the surface interpenetrating polymer network will beformed. The polymer is preferably any polymer that may be swelled by theabsorption of a monomer, with or without the aid of a solvent. Thepolymers used in the present invention are preferably selected from thegroup consisting of acrylics, acrylonitrile-butadiene-styrene copolymer,chlorinated polyvinylchloride, EPDM rubber, natural rubber, neoprene,nitrile rubber, polyethylene, polypropylene, polystyrene, polyurethanes,polyvinylchloride, silicones, thermoplastic elastomers; and vinylidenefluoride-hexafluoropropylene copolymer.

Generally, the surfaces of these polymers, in their untreated state, arehydrophobic. As used herein, hydrophobic means that the contact angle ofa drop of water on the surface of the polymer is greater than 90°. Acontact angle of less than 90° means that the surface of the polymer ishydrophilic. For instance, silicone has a typical contact angle of about100° to 110°. After undergoing the surface modification process of thepresent invention, the surface of the treated silicone has a contactangle of less than 90°, and may be as low as 40°. As used herein,contact angles and their measurement are described in Adamson, PhysicalChemistry of Surfaces, John Wiley & Sons, at pages 341-343 (1982).

The surface modification process of the present invention starts with acontrolled absorption of a swelling monomer into the surface of thepolymer substrate. The swelling monomer preferably comprises either atleast on crosslinking monomer, at least on functional monomer, or thecombination of at least one of each. The swelling monomer preferablycontains at least a di- or multi-functional agent. The swelling monomerused in accordance with the present invention is preferably chosen fromthe group consisting of acrylamides, methacrylamides, allylcrosslinkers, methacrylates, and vinyl crosslinkers.

The absorption of the swelling monomer into the polymer substrate may becarried out in the presence of a solvent. Even if no solvent is used,the swelling monomer may consist of one or more monomers, the monomersbeing either crosslinking monomers, functional monomers, or acombination of both. For each particular polymer substrate utilized inaccordance with the present invention, there is a correspondingpreferred type of solvent, as shown below in Table 1:

TABLE 1 Types of Preferred Solvents for Each Preferred Polymer SubstrateSUBSTRATE TYPE OF SOLVENT acrylics benzene and derivatives, chlorinatedhydrocarbons, alcohols, dioxane, ketones, acetic acid, acetates,isobutyric acid acrylonitrile- amines, anhydrides, ketones, DMF, DMSO,DMA, butadiene-styrene ethylene oxalate, ethylene carbonate, copolymer2-oxazolidone, cyanoacetic acid, sulfones chlorinated aromatichydrocarbons, chlorinated hydrocarbons, polyvinylchloride THF, dioxane,ketones, acetates, nitrobenzene, DMF, DMSO EPDM rubber hydrocarbons,halogenated hydrocarbons, aliphatic esters and ketones, ethers, acetatesnatural rubber benzene, halogenated hydrocarbons, hydrocarbons, THF,ketones, esters, ethers neoprene benzene, halogenated hydrocarbons,hydrocarbons, THF, ketones, esters, ethers nitrile rubber benzene,halogenated hydrocarbons, hydrocarbons, THF, ketones, esters, etherspolyethylene hydrocarbons, halogenated hydrocarbons, aliphatic estersand ketones, ethers, acetates polypropylene hydrocarbons, halogenatedhydrocarbons, aliphatic esters and ketones, ethers, acetates polystyrenehydrocarbons, chlorinated hydrocarbons, aromatic hydrocarbons, ethers,ketones, acetates polyurethanes phenol, m-cresol, formic acidpolyvinylchloride aromatic hydrocarbons, chlorinated hydrocarbons, THF,dioxane, ketones, acetates, nitrobenzene, DMF, DMSO siliconeshydrocarbons, aromatic and halogenated hydrocarbons, hydrogenatedxylene, 1,2-dimethoxyethane, phenetole, octylamine, ketones, acetatesthermoplastic hydrocarbons, halogenated hydrocarbons, esters andelastomers ketones, ethers, acetates vinylidene cyclohexanone,butyrolactone, fluoride- ethylene carbonate, DMA, N- hexafluoro-methylpyrrolidone, DMSO, hexafluorobenzene, propylene perfluorodibutylether, perfluorodibutylamine copolymer

The absorption of the swelling monomer into the polymer substrate in thepresence of a solvent should be sufficient to diffuse into and swell thepolymer substrate such that the swelling monomer will diffuse into thepolymer in a gradient. The swelling of the polymer by the swellingmonomer opens and exposes interslices in the polymer.

The polymer substrate is preferably swelled at a temperature of betweenapproximately −20° C. to 150° C. for a time period of betweenapproximately 0 to 96 hours.

In one embodiment of the present invention, the swelling monomer ispolymerized after the swelling step of the process has been completed.The swelling monomer is preferably polymerized in the presence of aninitiator. The types of initiators used, and the polymerization reactionconditions are discussed in more detail below.

In another embodiment of the present invention, the swollen polymer istransferred into an solution containing a functional monomer and aninitiator, after the polymer substrate has undergone the swelling step.This solution is also known as the interfacial reaction medium or thereaction mixture. The interfacial reaction medium may contain a solvent,but a solvent need not always be present.

It is within the interfacial reaction medium that the polymerizationreaction of the swelling monomer and functional monomer occurs. Thepolymerization reaction is preferably carried out a temperatures ofbetween approximately −78° C. to 150° C. for a period of time betweenapproximately 10 seconds to 72 hours.

A number of different functional monomers may be used in accordance withthe present invention to polymerize with the swelling monomer.Functional monomers having at least one amine, hydroxyl, or carboxylgroups are preferred. For example, the preferred functional monomers arechosen from the group consisting of acrylamides, methacrylamides,acrylates, methacrylates, allyl monomers, vinyl monomers, and styrenicmonomers.

In addition to the monomers listed above, oligomers or polymers havingsimilar polymerizable functions to the monomers listed above may also beused in accordance with the present invention.

If necessary, the polymerization reaction with the function monomer (or,as discussed above, the polymerization of the swelling monomer) isinitiated by a form of an initiator. Some of monomers and crosslinkersutilized in the process are thermo- or photo-sensitive. In processesusing these thermo- or photo-sensitive monomers and/or crosslinkers, thepolymerization can be initiated with ultraviolet light or heat. Further,the polymerization reaction may be initiated by ionizing radiation. Inother processes, the polymerization is initiated by a catalystinitiator. For monomers where a catalyst initiator is used, the catalystinitiator is preferably present in the interfacial reaction medium in aconcentration from between approximately 0.01% to 10%.

A solvent may also be present in the interfacial reaction medium. Thesolvent used in the interfacial reaction medium is preferably thesolvent indicated above in Table 1 for the specific polymer substratebeing used. The solvent provides the benefit of being able to increasethe depth of the interpenetrating polymer network in the polymersubstrate.

Upon initiation, polymerization proceeds in the solution, creating asoluble polymer. On the polymer surface, however, the polymerizationresults in an interpenetrating polymer network due to the presence ofthe crosslinker. The mutual solubility of the two solvents must be low,so that the polymerization takes place only at the surface/interface ofthe solvent systems.

In cases where a catalyst initiator is utilized (i.e. where thefunctional monomer and/or swelling monomer are not thermo- orphoto-sensitive and where ionizing radiation in not utilized), theinitiator is preferably chosen from the group consisting ofazo-initiators, peroxide initiators, and UV/visible initiators.

The following are exemplary of the process of the present invention forforming the surface interpenetrating polymer network on a polymersubstrate:

EXAMPLE 1

A silicone sheet is swollen in ethylene glycol dimethacrylate for 15hours at room temperature. The sheet is then transferred into an aqueoussolution containing 2-aminoethyl methacrylate hydrochloride (20%) and2,2′-azobis(2-methylpropionamidine) dihydrochloride (0.5%). The reactionis carried out at 60° C. for 1.5 hours. A contact angle of 72° isobtained on the modified silicone sheet.

EXAMPLE 2

A silicone sheet is swollen in bis(2-methacryloxyethyl) phosphate for 15hours at room temperature. The sheet is then transferred into an aqueoussolution containing 2-aminoethyl methacrylate hydrochloride (20%) and2-hydroxy-2-methyl-1-phenylproponone (1%). The reaction is carried outat room temperature for 10 minutes with UV radiation (366 nm, mediumintensity). A contact angle of 58° is obtained on the modified siliconesheet.

EXAMPLE 3

An AcrySof intraocular lens (a brand of soft acrylic lens made by AlconLaboratories), having a contact angle of 47°, was immersed inbis(2-methacryloxyethyl) phosphate for 1 hour, then in water for 20minutes. The lens was then placed in an aqueous solution containing 20%2-aminoethyl methacrylate hydrochloride and 1%2-hydroxy-2-methyl-1-phenyl-1-propanone. The lens containing solutionwas irradiated with 366 nm UV for 10 minutes. The contact angle wasmeasured at 56° before extraction and 72° after extraction.

In each of the three above-mentioned examples, the silicone substratematerial that has undergone the processes is ready for heparinization.

The surface interpenetrating polymer networks created by the process ofthe present invention provide numerous benefits. For instance, theprocess provides the benefit of versatility in that monomers withdifferent functionalities may be used to form the polymer network, suchas those with at least one amine, hydroxyl, and/or carboxyl group. Thus,these groups can become a part of the surface of the polymer, therebypermitting adhesion to these added groups by a surface modificationagent, such as the adhesion of heparin to an amine group at the surfaceof the polymer.

Further, the surface modification of the present invention providesbenefits over the traditional surface modifications. Traditionally, acoupling reagent is bonded directly to the surface of the polymersubstrate, as shown in FIG. 1. Thus, there is a covalent bond betweenthe coupling agent and one of the reactive groups on the surface of thepolymer substrate. The reaction of the coupling agent with the reactivegroup of the polymer substrate will require certain reaction conditionsdepending on the coupling agent and on the polymer substrate. In otherwords, the surface modification i substrate sensitive, because thereaction conditions and the coupling agent will vary depending on thepolymer substrate used. The traditional method id relatively stable, inthat the method provides a covalent bond between the coupling agent andthe polymer substrate surface which may be difficult to break. If,however, this one covalent bond between the coupling agent and thesurface of the polymer substrate is broken, then that coupling agentmolecule is lost as a surface.

In the present invention, the polymers of the surface interpenetratingnetwork do not bond to the polymer substrate. Instead, the polymerindirectly bonds with the substrate, through caternary connection andother forms of chain entanglement. Since the coupling agent does notbond directly to the substrate, the interpenetrating polymer network israther insensitive to the substrate surface, as shown in FIG. 2.Further, the surface interpenetrating polymer network of the presentinvention is also quite stable. In order to break the interpenetratingpolymer from the surface of the polymer substrate, a covalent bond inthe interpenetrating polymer itself must be broken. If such a bond isbroken, however, the remainder of the interpenetrating polymer willremain entangled with the polymer substrate. Thus, the surfaceinterpenetrating polymer network of the present invention is even morestable than the traditional method for surface modification, because,even if a covalent bond is the interpenetrating polymer is broken, theremainder of the interpenetrating polymer will remain entangled withinthe polymer substrate and be available as a surface modification.

The stability of the interpenetrating polymer network has been proven ina substrate through the use of attenuated total reflectance infrared orATRIR. ATRIR reveals the surface structure of a material. For instance,FIG. 3 shows the ATRIR spectra for an unmodified silicone substrate foruse in the present invention. These are the IR absorption peaks for thesilicone substrate.

FIG. 4 is a graph which shows the ATRIR for the silicone substrate afterit has undergone the interpenetrating polymer network process of thepresent invention. The IR absorption peaks for the silicone followingthe process of the present invention shows not only the substrate peaks,but also a new peak at about 1725 cm⁻¹, which is the carbonyl stretchingpeak of the interpenetrating polymer network.

In order to prove the stability of the interpenetrating polymer networkon the silicone substrate, the treated silicone substrate shown in FIG.4 was extracted with ethanol for three days at room temperature. TheATRIR for the extracted silicone substrate with the interpenetratingpolymer network is shown in FIG. 5. The carbonyl stretching peak of theunextracted silicone shown in Graph 2 is still present in FIG. 5 for theextracted silicone substrate. The continued presence of this peak in theATRIR indicates the permanent nature of the interpenetrating polymernetwork in the silicone substrate.

Anther benefit of the surface modification of the present invention isthat the surface modification may be coupled with a surface modificationagent, such as heparin. Heparinization of the polymer substrate havingthe surface interpenetrating polymer network of the present inventionmay be carried out as shown in the following example.

EXAMPLE 4 Heparinization of a Substrate Having an InterpenetratingPolymer Network

In this example, the heparin solution is prepared as follows. To 100 mlof deionized water, 0.25 g of heparin sodium and 1.0 g of citric acidare added. The pH is adjusted to between approximately 2 and 5, but 3.8is preferred, with 5M NaOH. Added to the heparin solution is 0.016 g ofNaCNBH3. The polymer substrate material that has previously been treatedwith the interpenetrating network process of the present invention isthen added to this solution. The reaction is carried out at 50° C. for 2hours. The polymer material is then rinsed with deionized water and0.025M sodium borate solution. The polymer substrate has now beenheparinized.

The polymer substrate may then be heparinized a second time. The polymermaterial that was treated with the first heparinization is placed in a0.025M sodium borate solution with 0.06% polyethylenimine for 1 hour atroom temperature. The material is removed and rinsed with deionizedwater. The material is then transferred back to the heparin solution andthe reaction is carried out at 50° C. for 3 hours. The samples are thenrinsed with 0.025M sodium borate solution and deionized water. After thesecond heparinization, the material has a typical contact angle of 20°.

The surface interpenetrating polymer network of the present inventiondiffers from and provides benefits over a bulk interpenetrating polymernetwork. For instance, in a bulk interpenetrating polymer network, theinitiator for the bulk polymerization is distributed in the bulk of thepolymer. In the surface interpenetrating polymer network of the presentinvention, however, the initiator is another phase, i.e., an aqueousphase, and thus the initiator is limited to the surface of the polymer.A polymer treated with a bulk interpenetrating polymer network hasdifferent properties than an untreated polymer, because the polymer hasa second polymer network in its bulk. These different properties, whichmay occur in the bulk process, have their drawbacks in that thesedifferent bulk properties may cause a polymer such as silicone to becomecloudy or hard, which reduce its ability to be used for opticalpurposes. A polymer treated with the surface interpenetrating polymernetworks of the present invention, however, maintains its bulkproperties, because the only modification occurs at the surface. Thus, asilicone polymer having a surface interpenetrating polymer network maybe sufficiently transparent for optical applications due to the thinnessof the surface interpenetrating polymer network. Moreover, in the bulkinterpenetrating polymer network, the functional monomer is mixed in thebulk and thus only part of the functional monomers are available forfunctioning on the surface the bulk interpenetrating polymer network. Onthe other hand, in the surface interpenetrating polymer network of thepresent invention, the functional monomers are available for functioningon the surface of the polymers. This is due to the fact that thefunctional monomer is in the aqueous phase during reaction and thusattach only to the polymer surface during polymerization and remain onthe outer layer of the polymer following reaction.

Numerous useful articles may be formed using the process of the presentinvention. For instance, one useful application of the process is thesurface modification of a silicone intraocular lens, which permits theheparinization of the lens surface. Further, a silicone contact lens ora silicone particle for use in a chromatography column may be made usingthe surface interpenetrating polymer process of the present invention.

While a particular form of the invention has been described, it will beapparent that various modifications can be made without departing fromthe scope of the invention. Accordingly, it is not intended that theinvention be limited by the specific embodiment disclosed in thedrawings and described in detail hereinabove.

1. A process for the surface modification of a silicone substrate, theprocess comprising the steps of: a. absorbing ethylene glycoldimethacrylate into silicone for between approximately 0.1 hours to 72hours at a temperature of between approximately 0° C. and 100° C. inorder to swell the silicone; b. removing the swollen silicone from theethylene glycol dimethacryate; c. transferring the swollen silicone intoan aqueous solution containing 2-aminoethyl methacrylate hydrochloridein a concentration of between approximately 0.1% and 50% and2,2′-azobis(2-methylpropionamidine) dihydrochloride in a concentrationof between approximately 0.1% and 10%; d. contacting the swollensilicone with the 2-aminoethyl methacrylate hydrochloride and the2,2′-azobis(2-methylpropionamidine) dihydrochloride at a temperature ofbetween approximately 30° C. and 80° C. for between approximately 0.1hours and 24 hours; and e. removing the silicone from the aqueoussolution.
 2. A process for forming a surface interpenetrating polymernetwork on a silicone substrate, the process comprising the steps of: a.absorbing bis(2-methacryloxyethyl) phosphate into silicone for betweenapproximately 0.1 hours and 72 hours at room temperature in order toswell the silicone; b. removing the swollen silicone from thebis(2-methacryloxyethyl) phosphate; c. transferring the swollen siliconeinto an aqueous solution containing 2-aminoethyl methacrylatehydrochloride in a concentration of between approximately 0.1% to 50%and 2-hydroxy-2-methyl-1-phenylpropanone in a concentration of betweenapproximately 0.1% and 10%. d. contracting the swollen silicone with the2-aminoethyl methacrylate hydrochloride and the2-hydroxy-2-methyl-1-phenylpropanone at a temperature of betweenapproximately 30° C. and 80° C. for between approximately 1 minute to 10hours with UV radiation; and e. removing the silicone from the aqueoussolution.
 3. A silicone contact lens having the surface modificationformed by the process of claim
 1. 4. A silicone contact lens having thesurface modification formed by the process of claim 2.